This invention is directed to a composite plastic knob which is attachable to a threaded shaft. Conventional plastic knobs used in the appliance industry, such as washer timer knobs, are frequently attached to a threaded shaft, which shaft is usually made of metal. Because such shafts are both moved longitudinally, that is, in and out, and rotated during normal operation of an appliance, the knobs are subjected to relatively severe longitudinal forces and torques. The longitudinal forces are caused by the inwardly pushing and outwardly pulling of the knob and its shaft. The torque is due to the turning of the knob and its shaft. To stand up to these relatively severe operating conditions, it has been conventional to equip such plastic knobs with molded in threads. However, the molded in threads often crack with the cracks propagating to such an extent that parts of the knob become separated from the knob body rendering the knob non-functional. Threaded metal inserts can be molded into or otherwise be assembled as part of a molded knob, however, such metal inserts are inordinately expensive. Threaded molded plastic inserts are more economical to manufacture than metal inserts but they are also prone to crack propagating problems as are molded in threads.
Accordingly, an object of this invention is a molded plastic threaded insert which limits crack propagation by providing an interface between an insert and a knob which eliminates the homogeneity of material that occurs when threads are molded into the knob.
Another object of this invention is a composite plastic knob having a threaded insert that resists pullout or push through even if the insert fractures during use.
Another object of this invention is a composite plastic knob in which the insert is formed of a plastic and is equipped with ribs which become embedded in the plastic of the knob body to resist pullout, push through or twisting of the insert relative to the knob body.
Yet another object of this invention is an insert for a composite knob which insert has triangularly shaped ribs formed with forward and rearward facing surfaces inclined to the longitudinal axis of insert so as to provide improved gripping with the plastic of the knob body.
Still another object of this invention is a composite plastic knob having a body molded to substantially encapsulate a threaded insert, the threaded insert having a knob body engaging surface of ribs and slots, the volume and number of the ribs and slots being adjustable to vary the mass of the molded knob body to avoid sink marks on the knob face while preventing push through of the insert.
Other objects of this invention will be found in the following specification, claims and drawings.